Ultra-wideband signal receiver using frequency sub-bands

ABSTRACT

A signal receiver suitable for digitising a signal having a wide bandwidth comprises a filter bank ( 30 ) for dividing a received signal into a plurality of frequency sub-bands, means ( 41 - 45 ) for digitising each sub-band using a low sample rate, means ( 51 ) for transforming each digitised sub-band signals into the frequency domain, means ( 61 - 65, 70 ) for concatenating the frequency domain sub-band signals to reconstruct the spectrum of the received signal. For a signal that occupies only one sub-band at any one instant, for example a frequency hopping signal or a chirp signal, a single analogue-to-digital converter may be used to digitise each sub-band in turn, and the transformation into the frequency domain may be performed for each sub-band in turn.

The present invention relates to a signal receiver and in particular,but not exclusively, to a signal receiver suitable for receiving awireless signal having a wide bandwidth.

The use of digital signal processing techniques to implement at leastthe baseband processing of a wireless receiver can bring benefits suchas increased versatility and decreased cost. Commonly, an RF signal ismixed down to a low IF or to baseband before digitisation becausedigitisation at a high frequency requires a high speedanalogue-to-digital converter (ADC). If conventional low-pass samplingis used to sample at the Nyquist rate, an ADC is required that cansample at twice the highest frequency of the signal. The sampling raterequired for digitisation at RF may exceed the capability ofcommercially available ADCs, or require an ADC having a high powerconsumption or a high cost. Even after mixing down to a lower frequency,sampling at the Nyquist rate for the signal may exceed the capability ofcommercially available ADCs, or require an ADC having a high powerconsumption or a high cost.

Ultra-wideband is a technique for performing radio communication andradio positioning which relies on sending a signal comprisingultra-short pulses. Such ultra-short pulses typically occupy frequenciesfrom zero to one or more GHz and it is not practical to use a single ADCwhich will digitise at the Nyquist rate a signal containing such highfrequencies. One solution for reducing the sampling rate below theNyquist rate for the signal is reported in “Ultra-wideband radartechnology”, edited by J. D. Taylor, CRC Press, 2001, pages 77-78. Thissolution uses a bank of filters to separate the frequency spectrum ofthe signal into several sub-bands, mix each sub-band down to DC, and todigitise each sub-band separately using a bank of ADCs, each ADCsampling at a rate equal to the highest frequency in the mixed-downsub-band. A block schematic diagram of this prior art solution isillustrated in FIG. 1 which shows a portion of a receiver suitable for asignal having a spectrum 0-1 GHz. The bank of filters, referred to as achannel dropping filter, divides the received signal into five sub-bandseach 200 MHz wide, and four mixers and four different local oscillatorsignals are required to bring the signal in all the sub-bands into therange 0-200 MHz where a bank of five ADCS, each ADC sampling at 200 MHz,is used to digitise the signal in the sub-bands. The digitised outputfrom each ADC is used to reconstruct the signal waveform, although nomethod or apparatus for doing this is disclosed in the Taylor reference.A disadvantage of this solution is the requirement for a bank of mixersand a means for generating a different local oscillator signal for eachmixer. Such a plurality of devices adds to complexity and cost because,for example, mixers are active devices that consume power, contributenoise and have a limited dynamic range.

A further solution is disclosed in “A Channelised DSSS Ultra-WidebandReceiver”, Won Namgoong, Proceedings RAWCON 2001, 2001 IEEE Radio andWireless Conference, Waltham Mass., USA, 19-22 Aug. 2001, pages 105-108.This solution uses a bank of mixers each requiring a different localoscillator signal and with each mixer followed by a sub-band filter, sosuffers the same disadvantages.

An object of the present invention is to provide a signal receiver thatovercomes at least some of the disadvantages of the prior art describedabove.

According to the invention there is provided a signal receivercomprising digitisation means for digitising a received signal anddemodulation means for extracting the information content of thedigitised received signal, wherein the digitisation means comprisesfiltering means for dividing the received signal into a plurality offrequency sub-bands, analogue-to-digital conversion means for digitisingthe signal in each sub-band, transform means for transforming thedigitised signal in each sub-band into the frequency domain, andreconstruction means for concatenating in the frequency domain thedigitised signal in each sub-band thereby reconstructing the spectrum ofthe received signal.

The reconstructed spectrum of the received signal may be at the samefrequency as spectrum of the received signal prior to being divided intosub-bands, or may be at a lower frequency, for example a received signalhaving a bandpass spectrum may be shifted to DC.

By dividing the received signal into a plurality of sub-bands anddigitising the signal in each sub-band, the requirement for an ADCcapable of digitising the whole signal bandwidth by operating at theNyquist rate is avoided. By digitising each sub-band without firstdownconverting each sub-band, the requirement for a bank of mixers andmeans for generating a different local oscillator signal for each mixeris avoided.

Optionally the received signal, if a bandpass signal, may bedown-converted from a transmission frequency to a lower frequency priorto the digitisation means.

The sample rate required for digitising the signal in a sub-band dependson the bandwidth of that sub-band, as described below.

By judicious selection of sample rate for each sub-band, it is possibleto use a common sample rate for a plurality of sub-bands, therebysimplifying the sample rate clock generation. Such sub-bands may have acommon bandwidth.

By judicious selection of sample rate and bandwidth for each sub-band,it is possible to use a common sample rate for odd numbered sub-bandsand a different common sample rate for even numbered sub-bands therebysimplifying the sample rate clock generation to two rates.

The analogue-to-digital conversion means may comprise an ADC for eachsub-band. However, if the receiver is to be used to receive a signalthat occupies only one sub-band at any one instant, such as a frequencyhopping signal or a chirp signal, the analogue-to-digital conversionmeans need only digitise one sub-band at a time and a single ADC can beswitched to each sub-band in turn, tracking the frequency of thereceived signal, thereby reducing the complexity of theanalogue-to-digital conversion means.

The invention will now be described, by way of example only, withreference to the accompanying drawings wherein:

FIG. 1 is block schematic diagram of a prior art receiver,

FIG. 2 is a diagram of the frequency response of a filter bank,

FIG. 3 is block schematic diagram of a first embodiment of a wirelessreceiver in accordance with the invention,

FIG. 4 is block schematic diagram of a second embodiment of a wirelessreceiver in accordance with the invention, and

FIG. 5 is a sketch illustrating sub-band spectra.

Referring to FIG. 3 a first embodiment of the invention comprises asignal input 5 for receiving a signal from an antenna. Coupled to thesignal input 5 is a low pass filter 10 for removing unwanted signalcomponents from the received signal. Coupled to the output of the lowpass filter 10 is an amplifier 20. The output of the amplifier 20 iscoupled to an input of a filter bank 30. The filter bank 30 divides thesignal delivered by the amplifier 20 into five sub-bands and the signalsin each sub-band are delivered to respective ADCs 41-45 where they aredigitised. Coupled to an output of each ADC is a respective FFT means51-55 for transforming the digitised sub-band signal to the frequencydomain, illustrated by the sketch 59 of a sub-band spectrum, using aFast Fourier Transform (FFT). Coupled to an output of each FFT means51-55 is a respective frequency shifting means 61-65 which performs ashifting of the frequency of the frequency domain sub-band signals to bepositioned at DC, illustrated by the sketch 69 of a shifted sub-bandspectrum, unless already at DC, by re-labelling of the frequencies.Coupled to an output of each frequency shifting means 61-65 is arespective storage portion 71-75 of a storage means 70. The frequencyshifted sub-band signals are concatenated by storing them in theirrespective storage portions 71-75, thereby reconstructing the receivedsignal in the frequency domain. An output of the storage means 70 iscoupled to a first input of a multiplier means 80 which multiplies thereconstructed received signal by a reference signal, the referencesignal being a replica of the signal spectrum as transmitted, beingstored in a reference signal store 85 and being delivered to a secondinput of the multiplier means 80. An output of the multiplier means 80is coupled to a means 90 for performing an Inverse Discrete FourierTransform (IDFT) which provides on an output 95 the cross-correlationfunction of the received signal and the reference signal. The output 95is coupled to a processing means (PROC) 96 for further processing of thecross-correlation function as required by the application for which thewireless receiver is to be used. For example, the time of occurrence ofa peak in the correlation function may be measured to determine theflight time of the signal, and consequently the range of the transmitterfrom the receiver. As another example, the polarity of a peak in thecorrelation function may be determined to detect the value of a data bitconveyed by the signal. As a further example, modulation of the time ofarrival of the received signal may be determined as conveyinginformation.

FIG. 2 shows the frequency response of a filter bank which divides asignal having a bandwidths into N sub-bands. In FIG. 2, each sub-bandhas the same bandwidth f_(b)/N but this is not essential. For theembodiment shown in FIG. 3, N=5.

If the signals in the sub-bands are to be sampled by the respective ADCs41-45 without aliasing, the ADC sampling rates must be selected tosatisfy the following inequality: $\begin{matrix}{\frac{2\quad f_{u_{i}}}{r_{i}} \leq f_{s_{i}} \leq \frac{2\quad f_{l_{i}}}{r_{i} - 1}} & (1)\end{matrix}$i=1 . . . N, where f_(s) _(i) is the sample rate for the i^(th)sub-band, f_(u) _(i) is the upper frequency limit of the i^(th)sub-band, f_(t) _(i) is the lower frequency limit of the i^(th)sub-band, and r_(i) is an integer satisfying the inequality$1 \leq r_{i} \leq {{int}\quad{\left\{ \frac{f_{u_{i}}}{f_{u_{i}} - f_{l_{i}}} \right\}.}}$In general, it is preferable that ADC sampling rates are selected suchthat aliasing is avoided; although for some types of received signal anamount of aliasing may be tolerable. In the following description it isassumed that aliasing is avoided.

Optionally r_(i) may be selected such that a plurality of sub-bandsshare a common sample rate, which simplifies sample rate clockgeneration. For example, for a 5-sub-band embodiment as shown in FIG. 3,and denoting the bandwidth of the i^(th) sub-band as W_(i), whereW_(i)=f_(u) _(i) −f_(t) _(i) , givessub-band 1: 2W ₁≦f_(s) _(i) ≦∞ for r₁=1,   (2)sub-band 2: 2(W ₁ +W ₂)≦f_(s2)≦∞ for r₂=1,   (3)sub-band 3: (W ₁ +W ₂ +W ₃)≦f_(s3)≦2(W ₁ +W ₂) for r₃=2,   (4)sub-band 4: (W ₁ +W ₂ +W ₃ +W ₄)≦f_(s4)≦2(W ₁ +W ₂ +W ₃) for r₄=2,   (5)sub-band 5: $\begin{matrix}\begin{matrix}{{{\frac{2}{3}\left( {W_{1} + W_{2} + W_{3} + W_{4} + W_{5}} \right)} \leq f_{s5} \leq \left( {W_{1} + W_{2} + W_{3} + W_{4}} \right)}\quad} \\{{{for}\quad r_{5}} = 3}\end{matrix} & (6)\end{matrix}$In this case, a common sample rate satisfying inequality (3) may be usedfor sub-bands 1 and 2.

Other possibilities of common sampling rates may be determined forspecific values of r_(i) and sub-band bandwidth W_(i).

Sub-bands sharing a common sample rate may optionally have a commonbandwidth. For example, if W₁=W₃=B, a common sampling rate can be usedfor sub-bands 1 and 3 provided that it satisfies the inequality3B≦f₃≦4B. As another example, if W₂=W₄=B, a common sampling rate can beused for sub-bands 2 and 4 provided that it satisfies the inequality 4B≦f₄≦6B.

Particularly advantageous sample rates can be derived by setting$r_{i} = \left\lfloor \frac{i + 1}{2} \right\rfloor$and W_(i)=B for all values of i, i.e. a common bandwidth for allsub-bands. In this case only two different sample rates are required; afirst common sample rate may be used for all the odd numbered sub-bands,provided that it satisfies the inequality (1) for the highestodd-numbered sub-band, and a second, different common sample rate may beused for all the even numbered sub-bands, provided that it satisfies theinequality (1) for the highest even-numbered sub-band. For theembodiment shown in FIG. 3 and a common bandwidth of B=200 MHz for allsub-bands, the first common sample rate must satisfy the inequality${{\frac{10}{3}B} \leq f_{s_{5}} \leq {4B}},$and so a value of 733 MHz would conveniently lie in the centre of theallowable range, and the second, different common sample rate mustsatisfy the inequality 4B ≦f_(s) _(i) ≦6B, and so a value of 1 GHz wouldconveniently lie in the centre of the allowable range.

Other possibilities of a first common sampling rate for the odd-numberedsub-bands and a different common sample rate for the even numberedsub-bands may be determined for specific values of sub-band bandwidthW_(i) where the sub-band bandwidths are unequal.

The acceptable range of values of the sample rate as expressed by theinequality (1) defines the acceptable limits of frequency error thateach sample rate may have. It is preferable to select sample rates thatlie approximately in the centre of their respective acceptable range.For the embodiment shown in FIG. 3 and with B=200 MHz, the tolerance ona first common sample rate of 733 MHz is approximately ±9%, and thetolerance on a second common sample rate of 1 GHz is ±20%.Alternatively, for a chosen sample rate f_(s) _(i) , the inequality (1)can be used to define the acceptable tolerance limits on f_(l) _(i) ,and f_(u) _(i) for the respective sub-bands provided by the filter bank30. Of course the tolerance on the sample rate may be traded fortolerance on f_(l) _(i) and f_(u) _(i) .

The digitised sub-band signals delivered by the ADCs 41-45 includecopies of the sub-band spectrum replicated at integer multiples of thesample rate. Therefore, the FFT means 51-55 comprise filtering means forselecting a single sub-band spectrum. FIG. 5 illustrates sub-bandspectra (amplitude A versus frequency f) for the example of a 0-1 GHzsignal divided into sub-bands 200 MHz wide.

FIG. 5 plot 5(a) in illustrates the first sub-band spectrum at 0-200 MHzwith a sample rate of f_(s) _(i) =733 MHz. Replica spectra are all above200 MHz and are filtered out by the FFT means 51; they are not shown inplot (a).

Plot (b) illustrates the second sub-band spectrum at 200-400 MHz with asample rate of f_(s) ₂ =1 GHz . Replica spectra are all above 400 MHzand are filtered out by the FFT means 52; they are not shown in plot(b).

Plot (c) illustrates the third sub-band spectrum at 400-600 MHz sampledat a sample rate of 733 MHz. The sampling process generates a replica ofthe sub-band spectrum at 133-333 MHz, reversed such that the lowerfrequencies of the sub-band prior to sampling now appear as the upperfrequencies of the replica at 133-333 MHz. In FIG. 5, the envelopes ofthe replica sub-band spectra are indicated with a broken line. The FFTmeans 53 comprises means for selecting the replica sub-band spectrum at133-333 MHz and also for reversing its spectrum to restore the order ofits frequency components.

Plot (d) illustrates the fourth sub-band spectrum at 600-800 MHz sampledat a sample rate of 1 GHz. The sampling process generates a replica ofthe sub-band spectrum at 200-400 MHz, reversed such that the lowerfrequencies of is the sub-band prior to sampling now appear as the upperfrequencies of the replica at 200-400 MHz. The FFT means 54 comprisesmeans for selecting the replica sub-band spectrum at 200-400 MHz andalso reversing its spectrum to restore the order of its frequencycomponents.

Plot (e) illustrates the fifth sub-band spectrum at 800-1000 MHz sampledat a sample rate of 733 MHz. The sampling process generates a replica ofthe spectrum at 67-267 MHz without any reversal. The FFT means 55comprises means for selecting the replica sub-band spectrum at 133-333MHz.

The frequency shifting means 61-65 are used to shift each selectedsub-band spectrum to DC. For the example, as illustrated in FIG. 5, theshift required by sub-bands 2, 3, 4 and 5 are respectively 200 MHz, 133MHz, 200 MHz, and 67 MHz. The shifting can be achieved by re-labellingthe frequency of the spectral components. Optionally, the reversing ofreplica sub-band spectra may be performed by the frequency shiftingmeans instead of by the FFT means.

The received signal is reconstructed in the frequency domain byconcatenating the frequency shifted sub-band signals in the storagemeans 70.

The process of concatenation shifts the i^(th) sub-band signal, i=2, N,in the frequency domain to respective frequencies f_(l) _(i) =2, N whichin the example illustrated in FIG. 5 are 200, 400, 600 and 800 MHz. Thereconstructed spectrum is illustrated in FIG. 3 by the sketch 79 of thespectrum delivered at the output of the storage means 70. The resolutionof the FFT means 51-55 depends on the sample rate of the respective ADC41-45; a high sample rate results in frequency components more closelyspaced than a lower sample rate. Because the resolution of each the FFTmeans 51-55 is not equal, the frequency components of the reconstructedspectrum are not uniformly spaced. This non-uniformity is notillustrated in the sketch 79 of the reconstructed spectrum. Thereference signal stored in the reference signal store 85 is specified atthe same non-uniformly spaced frequency values as the reconstructedspectrum. The means 90 for performing an IDFT is able to operate withthe non-uniform spaced frequency values.

Referring to FIG. 4 which illustrates a second embodiment of theinvention, identical reference numerals have been used for elements thatare identical or similar to elements of the embodiment illustrated inFIG. 3; the differences of elements of FIG. 4 are described below. Theembodiment illustrated in FIG. 4 can be used if the receiver is to beused to receive a signal that occupies only one sub-band at any oneinstant, such as a frequency hopping signal or a chirp signal. A singleADC 41, FFT means 51 and frequency shifting means 61 is used. The inputof the single ADC 41 is switched to each output of the filter bank 30 inturn by means of a first commutating switch means 100, and output of thesingle frequency shifting means 61 is switched to each storage portion71-75 in turn by means of a second commutating switch means 101. Theswitching of the first and second commutating switch means 100, 101 issynchronised by a synchronisation means (SYNC) 99. The order in whichsub-bands are selected for processing is predetermined to match theknown frequency hopping sequence or known chirp profile of thetransmitted signal A clock generator (CLK) 98 generates the sample ratesrequired by the ADC 41 for digitising each sub-band in turn and aselection switch means 102 is synchronised by the synchronisation means99 to select the required sample rate for each sub-band. The selectionswitch means 102 in FIG. 4 provides for selection between two samplerates, but any required number of selections may be provided. Thesynchronisation means 99 is also coupled to the FFT means 51 tosynchronise the switching of the filtering and reversing functions asappropriate to the current sub-band signal.

If the switching of the first and second commutating switch means 100,101, the selection switch means 102, and the filtering and spectrumreversing of the FFT means 51 are not synchronised to the sub-band whichthe received signal occupies at any one instant, after one commutationcycle the storage means 70 will not contain a complete set of sub-bandsignals and so the received signal will not be fully reconstructed inthe storage means 70 and the cross-correlation function provided at theoutput 95 will exhibit weak correlation. The output 95 is coupled to aninput of the synchronisation means 99 which adjusts the phase of thecommutating switch means 100, 101, the selection switch means 102, andthe filtering and reversing of the FFT means 51 until a maximumcorrelation is exhibited at the output 95. Optionally, means fordetecting the signal strength in each sub-band may be included toprovide an indication to the synchronisation means 99 of the currentfrequency occupancy of the received signal, thereby assisting thesynchronisation means 99 to synchronise the commutating cycle, theselection switch means 102 and the FFT means 51 with the receivedsignal.

Although the invention has been described by means of an example of awireless receiver, the invention is equally applicable to a receiver forreceiving a signal via a different medium, for example via wire oroptically.

Although the invention has been described by means of an example of areceiver suitable for receiving a wide-band or ultra-wideband signal,the invention can also be used for receiving signals having a narrowerbandwidth.

Optionally a demodulation process different to that described herein maybe applied to demodulate the digitised received signal.

Optionally the receiver may comprise a power saving scheme in which someor all of the receiver elements adopt a power saving mode and areactivated at intervals to receive a signal. For example, if the receiveris to be used to receive a signal having a duty cycle less than one,then a bank of ADCs can also be operated with a duty cycle less thanone, sampling only for periods in which the signal is expected to bepresent.

In the present specification and claims the word “a” or “an” precedingan element does not exclude the presence of a plurality of suchelements. Further, the word “comprising” does not exclude the presenceof other elements or steps than those listed.

From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in the design, manufacture anduse of signal receivers, and which may be used instead of or in additionto features described herein.

1. A signal receiver comprising digitisation means for digitising areceived signal and demodulation means (80, 85, 90, 96) for extractingthe information content of the digitised received signal, wherein thedigitisation means comprises filtering means (30) for dividing thereceived signal into a plurality of frequency sub-bands,analogue-to-digital conversion means (41-45) for digitising the signalin each sub-band, transform means (51-55) for transforming the digitisedsignal in each sub-band into the frequency domain, and reconstructionmeans (51-55, 61-65, 70) for concatenating in the frequency domain thedigitised signal in each sub-band thereby reconstructing the spectrum ofthe received signal.
 2. A receiver as claimed in claim 2, wherein thereconstruction means (51-55, 61-65, 70) reconstructs the spectrum of thereceived signal at a frequency lower than the frequency of the spectrumof the received signal prior to being divided into sub-bands.
 3. Areceiver as claimed in claim 1 or 2, wherein the analogue-to-digitalconversion means (41-45) comprises means for sampling the signal in thei^(th) sub-band at a sample rate f_(s) _(i) in the range$\frac{2\quad f_{u_{i}}}{r_{i}} \leq f_{s_{i}} \leq \frac{2\quad f_{l_{i}}}{r_{i} - 1}$where f_(u) _(i) is the upper frequency limit of the sub-band and f_(l)_(i) is the lower frequency limit of the i^(th) sub-band, and r_(i) isan integer satisfying the inequality$1 \leq r_{i} \leq {{int}\quad{\left\{ \frac{f_{u_{i}}}{f_{u_{i}} - f_{l_{i}}} \right\}.}}$4. A receiver as claimed in claim 3, wherein the analogue-to-digitalconversion means (41-45) comprises means for sampling the signal in aplurality of the sub-bands at a common sample rate.
 5. A receiver asclaimed in claim 3, wherein the analogue-to-digital conversion means(41-45) comprises means for sampling the signal in a first sub-set ofthe sub-bands at a first sample rate and for sampling the signal in asecond sub-set of the sub-bands at a second sample rate and wherein thesignal in adjacent sub-bands is sampled at unequal sample rates.
 6. Areceiver as claimed in claim 4 or 5, wherein the plurality of sub-bandshaving a common sample rate have a common bandwidth.
 7. A receiver asclaimed in any one of claims 1 to 6, wherein the analogue-to-digitalconversion means comprises means 41 for digitising a plurality ofsub-bands sequentially.
 8. A receiver as claimed in claim 7, wherein thetransform means comprises means (51) for transforming the digitisedsignal in a plurality of the sub-bands sequentially.
 9. A receiver asclaimed in any one of claims 1 to 8, wherein the reconstruction meanscomprises means (51-55) for selecting a replica spectrum of a sub-bandsignal and means (51-55 or 61-65) for re-inverting the replica spectrumif the replica spectrum is inverted.
 10. A receiver as claimed in anyone of claims 1 to 9, wherein the demodulation means comprises means(80) for multiplying the reconstructed received signal by a referencesignal in the frequency domain at non-uniformly spaced frequencies. 11.A receiver as claimed in any one of claims 1 to 9, comprisingdown-conversion means prior to the digitisation means fordown-converting the received signal from a transmission frequency to alower frequency.